1. Field of the Invention
The present invention relates to a carbon nanotube array and a method for forming the same, and more particularly to a carbon nanotube array in which carbon nanotubes are well aligned and a method for forming the same.
2. Description of Prior Art
Carbon nanotubes are very small tube-shaped structures having the composition of a graphite sheet rolled into a tube. Carbon nanotubes produced by arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58).
Carbon nanotubes are electrically conductive along their length, are chemically stable, and can have very small diameters (much less than 100 nanometers) and large aspect ratios (length/diameter). Due to these and other properties, it has been suggested that carbon nanotubes can play an important role in fields such as microscopic electronics, materials science, biology and chemistry.
Existing carbon nanotube synthesis techniques include arc discharge, laser vaporization, and chemical vapor deposition (CVD). The arc discharge and laser vaporization methods are not able to satisfactorily control the diameter or length of the carbon nanotubes formed, and the yield of these methods is relatively low. Moreover, excess amorphous carbon lumps are also produced along with the carbon nanotubes, thus necessitating complicated purification processes. In summary, industrial production of carbon nanotubes using these methods is problematic. The chemical vapor deposition method is known in the art as being conducive to growing carbon nanotube arrays with well aligned carbon nanotubes.
A method for preparing a carbon nanotube array with well aligned carbon nanotubes is disclosed in an article by Fan, S. S. et al. entitled “Self-oriented regular arrays of carbon nanotubes and their field emission properties” (Science, Vol. 283, 1999, pp. 512-514). The method comprises the following steps: providing a porous silicon substrate with pore diameters of approximately 3 nanometers (nm); patterning the substrate with an iron (Fe) film by electron beam evaporation through shadow masks; annealing the substrate in air at 300° C. overnight; placing the substrate in a quartz boat and inserting the quartz boat into the center of a quartz tube reactor in a tube furnace; heating the furnace to 700° C. in flowing argon (Ar), and introducing flowing ethylene at 1000 standard cubic centimeters per minute (sccm) for 15 to 60 min.; and cooling the furnace to room temperature. A carbon nanotube array with well aligned carbon nanotubes can be observed as having been formed on top of the patterned iron squares on the substrate, by using a scanning electron microscope (SEM).
During the growth of a carbon nanotube, amorphous carbons are simultaneously deposited on the outer surface thereof. This considerably decreases van der Waals attraction between adjacent carbon nanotubes. By using the above-described method of Fan et al., van der Waals attraction between carbon nanotubes in the carbon nanotube array is relatively weak. FIG. 8 shows a transmission electron microscope (TEM) image of a carbon nanotube array formed by using the above-described method of Fan et al. and then by ultrasonicating the carbon nanotubes in 1,2-dichloroethane for 10 minutes. Carbon nanotubes in the carbon nanotube array are seen to be randomly distributed in the dichlorethane.